1. Field of the Invention
The present invention relates to a micro-electro mechanical systems (MEMS) capacitive sensor, and more particularly to an apparatus and a method for uniformizing output signal levels of MEMS capacitive sensors by compensating output signal level deviations caused by fabrication process variations without adjusting a process condition.
2. Description of the Related Art
MEMS stands for Micro-Electro Mechanical Systems and is defined as a technology combining very small-sized mechanical components such as sensors, valves, gears, mirrors and actuators implemented in a semiconductor chip and a computer.
Such MEMS has a wide range of applications such as navigation systems, air flow detecting sensors embedded in a flight wing for sensing air flow change in response to a surface resistance of a flight wing, an optical switching devices for aiding exchange of optical signals at 20 ns between separate optical signal paths, sensor actuating type air-conditioning systems, and sensors embedded in the base of building for changing characteristic of material by sensing air pressure. A magnetic sensor and a vibration accelerometer typically contained in an air bag of an automotive vehicle are the most representative applications of such MEMS.
Typically, an MEMS device is implemented by incorporating mechanical components such as mirrors and sensors, and microcircuits on a small silicon chip.
FIGS. 1A and 1B illustrate a conventional MEMS capacitive sensor. Referring to FIGS. 1A and 1B, the MEMS capacitive sensor 10 includes a glass substrate 11, silicon layers 12, 14 formed on the glass substrate 11, a microstructure 16 formed by etching silicon layers 12, 14 and a fixed electrode 17 spaced from the microstructure 16 by a certain gap. The MEMS capacitive sensor 10 operates to detect capacitance variation generated by displacement of the microstructure 16. The displacement is caused by a force of acceleration or the Lorentz force based on terrestrial magnetism. That is, as the displacement occurs, a size of the gap between the fixed electrode 17 and the microstructure 16 changes. The gap size variations cause a variation of capacitance between the microstructure and the fixed electrode. The MEMS capacitive sensor detects such capacitance variation.
Accordingly, in the MEMS capacitive sensors described above, an initial distance between the microstructure 16 and the fixed electrode 17 to must be constant so as to enable the MEMS capacitive sensors to output the same result under the same condition.
For example, with reference to FIG. 2A, in the case that springs 21, 22 supporting respective microstructures in different MEMS capacitive sensors have different widths Wsp_A and Wsp_B, respectively, that is, the width Wsp_A is smaller than the width Wsp_B, the modulus of rigidity of the spring 22 having relatively greater width Wsp_B is greater than that of the spring 21 having relatively smaller width Wsp_A. Since as the modulus of rigidity becomes greater, restoring force of the spring becomes smaller, the spring 22 having the relatively greater width Wsp-B has lesser restoring force than the spring 21.
Accordingly, with reference to FIG. 2B, assuming that an initial distance between a microstructure 24 supported by the spring 21 and a fixed electrode 23 is d0, with reference to FIG. 2C, an initial distance between a microstructure 26 supported by the spring 22 and a fixed electrode 25 deviates from the initial distance d0 to the extent of ±Δd because the restoring force of the spring 21 is less than that of the spring 22.
Such gap deviation causes a change in initial capacitance, thereby changing an output AC level of the MEMS capacitive sensor.
The gap deviation is caused by process variations existing whenever different semiconductor chips or wafers are processed.
Further, there are other causes of initial gap deviation: 1) gap status of a fixed electrode: etching depth or a degree of tilt, 2) wafer bonding status: interface grain or wafer inclination caused by wafer handling, and 3) bending caused by a stress and droop of a microstructure.
No matter how rigorously the fabrication process is controlled, such process variations may occur, and it is very difficult to find and remove the causes of such process variations.
Accordingly, an additional device or apparatus to compensate the output signal level deviations due to the process variations is required so as to make output signal levels of the MEMS sensors uniform.
FIG. 3 illustrates a conventional apparatus for making output levels of MEMS sensors uniform. A conventional apparatus for making output levels of MEMS sensors uniform is used for ensuring reproducible displacement of MEMS sensors under the same condition.
As shown in FIG. 3, a conventional output level uniform apparatus in the MEMS sensor includes sensing electrodes F1, F3 fixedly arranged at both end portions in one side of a microstructure F0 for sensing capacitance changes and control electrodes F2, F4 arranged at both end portions of the other side of the microstructure F0 for controlling displacement of the microstructure F0. The control electrodes F2, F4 are applied with equilibrium DC voltages to generate electrostatic force so that the microstructure F0 is allowed to positioned at the center of the sensing electrodes F1, F3.
However, the conventional output level uniform apparatus is disadvantageous in that a structure of the apparatus is complicated because it forcibly adjusts the location of the microstructure F0 by applying an additional electrostatic force to the microstructure F0. The conventional output level uniform apparatus further has a drawback that output levels may still vary due to the interference between the output signal and the electrostatic force caused by the control electrodes F2, F4.
Further, since the microstructure F0 is configured to be placed in the center of the sensing electrodes F1, F3, in the case that an initial gap size error of a gap between the sensing electrodes F1, F3 and the microstructure F0 exists, it is impossible compensate such initial gap deviation, so that output levels of all the microstructure may be different from each other.